1. Field of the Invention
The present invention relates to an image forming apparatus using a multi-beam scanner for performing simultaneous scanning with a plurality of beams. Particularly it relates to a multi-beam image forming apparatus in which image formation start positions can be aligned to thereby obtain a high-quality image even in the case where a large number of beams are used.
2. Description of the Related
In an electrophotographic apparatus such as a laser printer, a digital copying machine, etc., after a photoconductor drum is charged evenly, an electrostatic latent image is formed on the photoconductor drum in accordance with recording information by an exposure device using laser beams. The electrostatic latent image is developed with toner to form a toner image. The toner image is transferred onto a sheet of paper by a transfer unit. Further, the toner image is fixed to thereby form an image on the sheet of paper.
As this type of image forming apparatus, there has been heretofore proposed a multi-beam image forming apparatus having a multi-beam scanner using a polygon mirror for simultaneously scanning a plurality of line with a plurality of laser beams. This type of multi-beam image forming apparatus has such a characteristic that a low-speed rotation polygon motor and low-power semiconductor lasers can be used for forming an image at a high speed because an image corresponding to the plurality of lines is formed by one surface of the polygon mirror.
In the multi-beam image forming apparatus, alignment of image formation start positions of the plurality of laser beams is not only necessary for simultaneous recording of image data corresponding to the plurality of lines with the laser beams, but also essential for achievement of high image quality. For this reason, there is used a control method in which positions after a predetermined time distance from beam detection signals output from a laser beam detection unit that is disposed in a predetermined position out of an effective scanning range and is irradiated with laser beams are set as image formation start positions, prior to start of image formation.
When, for example, a plurality of laser beams are aligned in a main scanning direction, one of the laser beams is emitted and applied on a beam detection unit so that the image formation start positions of all the laser beams can be controlled on the basis of the beam detection signal output from the beam detection unit in the same manner as in an image forming apparatus using one laser beam. In this method, accuracy in synthesizing the plurality of laser beams is however limited. Even in the case where the plurality of laser beams can be synthesized accurately, it is inevitable that accuracy in synthesizing is lowered with the passage of time because of the influence of environmental change, vibration, etc. Accordingly, it is difficult to always align the image formation start positions of the laser beams in the main scanning direction stably.
To solve this problem, JP-A-10-202943 discloses a method for controlling the image formation start positions of laser beams respectively.
FIG. 13 is a schematic view showing part of an image forming apparatus for explaining the aforementioned method. In FIG. 13, the reference numeral 1 designates a polygon mirror; 2, an imaging lens system; 3, a mirror; 4, a photoconductor drum; 5, a synchronous detection unit; and 6, a laser control unit.
LD1 and LD2 designate first and second semiconductor laser beam sources respectively. Laser beams emitted from the two laser beam sources LD1 and LD2 are reflected by a deflection/reflection surface of the polygon mirror 1 so as to be distributed horizontally. Then, the laser beams are changed into convergent beams by the imaging lens system 2 such as an fθ lens. The optical path of the laser beams is bent downward by the mirror 3. The laser beams are focused as two light spots S1 and S2 on the photoconductor drum 4. The photoconductor drum 4 is scanned with the two light spots S1 and S2 simultaneously in the main scanning direction to thereby form an electrostatic latent image.
The first and second semiconductor laser beam sources LD1 and LD2 are adjacently disposed in the main scanning direction so that the light spots S1 and S2 are formed so as to be separated from each other at a slight distance P corresponding to resolution as shown in FIG. 14. For this reason, the positions of the two light spots S1 and S2 for scanning the photoconductor drum 4 or the synchronous detection unit 5 are shifted by a distance L in the main scanning direction, so that a time difference corresponding to the distance L is generated when the two light spots S1 and S2 are incident on a light-receiving surface 5a of the synchronous detection unit 5.
Accordingly, when the semiconductor laser beam sources LD1 and LD2 are made to emit light at the timing exemplified in FIG. 15, synchronous signal outputs (A) and (B) corresponding to the light spots S1 and S2 are obtained from the light-receiving surface 5a of the synchronous detection unit 5. When the semiconductor laser beam sources LD1 and LD2 are controlled by the laser control unit 6 on the basis of the synchronous signals (A) and (B), the start positions of the laser beams in the main scanning direction can be always controlled stably.
In the control method, there is however a problem that the effective scanning range is narrowed when the semiconductor laser beam sources are formed as an array to increase the number of laser beams, for example, to ten laser beams.
That is, when ten laser beam spots S1 to S10 are shifted at regular intervals in the main scanning direction and the sub-scanning direction orthogonal to the main scanning direction as shown in FIG. 16, scanning due to the laser beams is expressed as shown in FIG. 17. The effective scanning range of the laser beams is decided as a predetermined range R1 from the image formation start position X because the effective scanning range is a range in which all the scanned lines by the laser beam spots S1 to S10 overlap. Accordingly, if imaging is performed as shown in FIG. 16, the overlapping portion is reduced so that the problem of narrowing the effective scanning range cannot be avoided.
On the other hand, JP-A-6-344592 discloses a method in which two beam detection units are adjacently disposed in the main scanning direction so that the beam of the first semiconductor laser is detected by the first beam detection unit while the beam of the second semiconductor laser is detected by the second beam detection unit to thereby generate a synchronous signal (beam detection signal) as a reference signal for aligning the image formation start positions.
Also in this method, the scanning range for detecting the beams however becomes long because the plurality of beam detection units are adjacently disposed in the main scanning direction. As a result, there is a problem that the effective scanning range is narrowed.